1. Field of the Invention
The present invention relates to a method and apparatus for separating a composite member, separated members, and a semiconductor substrate and its production method.
2. Related Background Art
The formation of a single crystal Si semiconductor layer on an insulating surface of a substrate is widely known as a semiconductor on insulator (SOI) technique, and many efforts have been made to research this technique because devices produced using the SOI technique have many advantages that cannot be achieved by bulk Si substrates used to fabricate normal Si integrated circuits.
The use of the SOI technique provides the following advantages:
(1) The dielectric separation can be easily made to attain high integration.
(2) Radiation resistance is excellent.
(3) The stray capacity is reduced to attain high speed.
(4) The well formation process can be omitted.
(5) Latch-up can be prevented.
(6) The thickness can be reduced to provide a fully depleted field effect transistor.
To achieve the many advantages of the device, methods for forming SOI structures have been researched for decades. One of such known methods is SOS (silicon on sapphire) in which Si is heteroepitaxially formed by CVD (chemical vapor deposition) on a single crystal sapphire substrate. This technique has been successful as the maturest SOI technique, but its applications are limited by a large amount of crystal defects due to the misalignment of lattices in the interface between an Si layer and a sapphire substrate, by the mixture of aluminum from the sapphire substrate into the Si layer, and in particular, by the high costs of the substrate and the still insufficient the enlargement of area of the device. More recently, an attempt has been made to implement an SOI structure without the sapphire substrate. This attempt can be roughly classified into the following two methods.
1. After the surface of an Si single crystal substrate is oxidized, a window is made in the oxidized film to expose a part of the surface of the Si substrate, and this part is used as a seed to allow a horizontal epitaxial growth to form an Si single crystal layer on the SiO2 (in this case, an Si layer is deposited on SiO2).
2. The Si single crystal substrate is used as an active layer and SiO2 is formed under this layer (this method does not require an Si layer to be deposited).
Known means for realizing the above method 1 include a method for allowing the direct horizontal epitaxial growth of single crystal layer Si using CVD, a method of depositing amorphous Si and allowing its horizontal epitaxial growth in a solid phase by thermal treatment, a method of irradiating an amorphous or polycrystal Si layer with converging energy beams such as electron or laser beams, and allowing a single crystal layer to grow on SiO2 by means of melting recrystallization, and a method of using a bar-like heater to scan a molten area in such a way that the scanning trace appears like a band (zone melting recrystallization). Although these methods have both advantages and disadvantages, they still have many problems in terms of their controllability, productivity, uniformity, and quality and none of them have been put to industrially practical use. For example, the CVD method requires sacrificial oxidization to provide flat films. The solid phase growth method provides poor crystallinity. The beam anneal method has problems in terms of the time required for converging-beam scanning, and control of the superposition of beams, and focusing. Among them, the zone melting recrystallization method is maturest and has been used to produce relatively-large-scale integrated circuits on an experimental basis, but it still causes a large amount of crystal defects such as sub-grains to remain in the device, thereby failing to fabricate minor-carrier devices and to provide sufficiently excellent crystals.
The above method 2 that does not use the Si substrate as a seed for epitaxial growth includes the following four methods.
(1) An oxide film is formed on an Si single crystal substrate with a V-shaped groove etched anisotropically in its surface, a polycrystal Si layer is deposited on the oxide film so as to be as thick as the Si substrate, and then an Si single crystal region surrounded by the V-shaped groove so as to be separated dielectricly is formed on the thick polycrystal Si layer by polishing from the rear surface of the Si substrate. This method provides excellent crystallinity but the steps for depositing polycrystal Si by a thickness of several hundred microns and polishing the single crystal Si substrate from its rear surface to leave only the separated Si active layer have problems in terms of controllability and productivity.
(2) SIMOX (Separation by Ion-Implemented Oxygen) that forms an SiO2 layer in an Si single crystal substrate by means of oxygen ion implantation and that is the presently maturest technique due to its excellent compatibility with the Si process. To form an SiO2 layer, however, 1018 ions/cm2 or more of oxygen ions must be implanted, resulting in the need for a large amount of time for the implantation, thereby leading to reduced productivity. In addition, the costs of wafers are high. Furthermore, this method cause a large amount of crystal defects to remain in the device and does not industrially provide a sufficient quality to fabricate minor-carrier devices.
(3) A method for forming an SOI structure by dielectric separation through the oxidization of porous Si. In this method, an N-type Si layer is formed like an island on a surface of a P-type Si single crystal substrate by proton-ion implantation (Imai et al., J. Crystal Growth, vol. 63, 547 (1983)) or by epitaxial growth and patterning. Only the P-type Si substrate is made porous by an anodization method using an HF solution in such a way that the porous region surrounds the Si island from the surface, and the N-type Si island is then oxidized at a high speed for dielectric separation. In this method, the separating Si region is determined prior to the device step, thereby limiting the degree of freedom of device design.
(4) A method for forming an SOI structure using thermal treatment or an adhesive to bond an Si monocrystal substrate on a different Si single crystal substrate that is thermally oxidized is attracting attention. This method requires an active layer for a device to be formed as a uniformly thin film. That is, the thickness of a several-hundred-micron-thick Si single crystal substrate must be reduced to the order of micron or less.
The following two methods can be used to provide a thinner film.
1) Thickness reduction by polishing
2) Thickness reduction by selective etching
The polishing in 1) cannot provide uniformly thin films easily. In particular, if the thickness is reduced to the order of submicron, the thickness variation will be several tens %, resulting in a serious problem for providing uniformity. The difficulty in achieving uniformity further increases with increasing size of the substrate.
In addition, although the etching in 2) is supposed to be effective in providing uniform thin films, it has the following problems.
The selection ratio is at most 102 and is insufficient.
The surface obtained after etching is bad.
The crystallinity of the SOI layer is bad due to the use of ion implantation or epitaxial or heteroepitaxial growth on a high concentration B-doped Si layer.
A semiconductor substrate formed by bonding requires two substrates, one of which is mostly uselessly removed and disposed of through polishing and etching, thereby wasting limited global resources. Thus, SOI with bonding presently has many problems in terms of its controllability, uniformity, and costs.
In addition, generally due to the disorder of the crystal structure of a light-transmissive substrate represented by glass, a thin film Si layer deposited on the substrate can only form an amorphous layer or a polycrystal layer based on the disorder of substrates, and therefore high-performance devices cannot be produced. This is because since amorphous structure of the substrate is amorphous, an excellent single crystal layer cannot be obtained by simply depositing an Si layer. The light-transmissive substrate is important in producing a contact sensor or a projection liquid-crystal image display device that is a light-receiving element. Not only the improvement of pixels but also a high-performance drive element are required to attain higher density, higher resolution, and finer definition of the pixels in the sensor or display device. Thus, to provide elements on the light-transmissive substrate, a single crystal layer of an excellent crystallinity is required.
Among such SOI substrate production methods, the method of forming a non-single-crystal semiconductor layer on a porous layer and transferring the layer onto a supporting substrate via an insulating layer as disclosed in Japanese Patent Application Laid-Open No. 5-21338 is very excellent due to the uniform thickness of the SOI layer, its capability of maintaining the crystal-defect density of the SOI layer at a low level easily, the flatness of the surface of the SOI layer, no need for an expensive apparatus of a special specification for fabrication, and the capability of using the same apparatus for various SOI film thicknesses ranging from about several 100 Angstrom to 10 micron.
Furthermore, by combining the above method with the method disclosed in Japanese Patent Application Laid-Open No. 7-302889, that is, by forming a nonporous single crystal semiconductor layer on a porous layer formed on a first substrate, bonding the nonporous single crystal layer onto a second substrate via an insulating layer, separating the first substrate and the second substrate by the porous layer without destruction, and smoothing the surface of the first substrate and forming porous layer again for reuse, the first substrate can be used many times. This method can significantly reduce production costs and simplify the production steps.
There are several methods for separating the bonded substrates mutually to divide into the first substrate and the second substrate without destruction. For example, one of them is to pull the substrate in a direction vertical to the bonded surface. Another method is to apply a shearing stress in parallel with the bonded surface (for example, moving the substrates in the opposite directions within planes in parallel with the bonded surface or rotating the substrates in the circumferentially opposite directions). A pressure can be applied to the bonded surface in the vertical direction. Furthermore, a wave energy such as ultrasonic waves can be applied to the separation region. A peeling member (for example, a sharp blade such as a knife) can also be inserted into the separation region in parallel with the bonded surface from the side of the bonded substrates. Furthermore, the expansion energy of a material infiltrated into the porous layer that functions as the separation region may be used. The porous layer functioning as the separation region may also be thermally oxidized from the side of the bonded substrates to expand the volume of this layer. The porous layer functioning as the separation region may also be selectively etched from the side of the bonded substrates to separate the substrates. Finally, a layer formed by ion implantation to provide microcavities may be used as the separation region and the substrates may then be irradiated with laser beams from the normal direction of the bonded surface to heat the separation region containing the microcavity for separation.
However, these methods for separating the two bonded substrates mutually are ideally excellent, but all of them are not suitable for the production of semiconductor substrates. One of the difficulties is that the bonded semiconductor substrates are generally shaped like discs and have a small thickness, for example, 0.5 to 1.0 mm and that the bonded portion has few relatively large recesses on which a jig can be caught. Thus, a method of catching on an orientation flat portion of each substrate a jig having a recessed portion that fits the orientation flat portion and rotating the substrates in parallel with the bonded surface, or a method of catching the jig on a small recessed portion made in the bonded portion in the side of the bonded substrates to peel them are limited. The pressure-based separation requires a very large pressure, thereby forcing the size of the apparatus to be increased. In the wave energy method, the wave irradiation method must be substantially improved to irradiate the bonded substrates with wave energy efficiently, and immediately after separation, the separated substrates may partly contact and damage each other. In the separation from the side, the substrates may be bent to allow only their sides to be peeled, with their central portions remaining unseparated. In the method of inserting the peeling member into the separation region from the side of the bonded substrates, the insertion of the peeling member may damage the bonded surface between the substrates due to the friction of the peeling member and the substrates.
One solution for avoiding these problems is to reduce the mechanical strength of the separation region appropriately. This method, however, may increase the possibility that the separation region is destroyed by an external impact prior to the bonding of the substrates. In such a case, part of the destroyed separation region may become particles and contaminate the inside of the production apparatus. Although the conventional separation methods have the major advantages, they still have the above problems.
It is an object of this invention to provide an improved separation method and apparatus that can separate the bonded substrates mutually without destruction to prevent the separated substrates from being damaged and that is unlikely to destroy the separation region prior to the step of separating bonded substrates even when an external force is applied thereto, thereby preventing the production apparatus from being contaminated with particles.
The feature of this invention resides in that a composite member having a plurality of members as mutually bonded is separated into a plurality of members at positions different from the bonded position (separation region) of the plurality of members by jetting a fluid against the composite member.
With respect to the separation method, the composite member may be any member having a separation region inside, whereas with respect to the semiconductor substrate production method, it must have the following structure. A major example of the composite member is bonded substrates by bonding a first substrate and a second substrate, the first substrate being a semiconductor substrate in which a separation region is formed as a layer in a portion located deeper than its surface and in parallel therewith and in which the surface and the portion shallower than it has no separation region. That is, when this invention is applied to the semiconductor substrate production method, the members obtained after separation are not the same as the first and second substrates prior to bonding.
According to this invention, the separation region is located at a position different from the bonding interface (junction surface) between the first and second substrates. In the separation step, the substrates must be separated by the separation region located at the position different from the bonding interface.
Thus, the separation region is adapted to be mechanically weaker than the bonding interface so that the separation region is destroyed before the bonding interface. Thus, when the separation region is destroyed, a portion of the surface side of the first substrate which has a predetermined thickness is separated from the first substrate while remaining bonded on the second substrate, thereby transferring the portion to the second substrate. The separation region may be a porous layer formed by the anodization method or a layer formed by ion implantation to provide microcavities. These layers have a large amount of microcavities. This region may also be a heteroepitaxial layer in which distortion and defects are concentrated in crystal lattices.
The separation region may also be multiple layers of different structures. For example, it may consist of multiple porous layers having different porosities or a porous layer of a porosity changing in the direction perpendicular to the layers, as required.
The layer transferred from the first substrate to the second substrate by, for example, separating the composite member comprising the first and second substrates bonded together with each other via the insulating layer is used as a semiconductor layer (an SOI layer) on the insulating layer to fabricate a semiconductor device.
Jet of a fluid that can be used for the separation can be conducted by a so-called water jet method that injects a flow of high-pressure water through a nozzle. Instead of water, this fluid may be an organic solvent such as alcohol, an acid such as hydrofluoric or nitric acid, an alkali such as potassium hydroxide, or a liquid capable of selectively etching the separation region. A fluid consisting essentially of an abrasive particle-free liquid is preferable. Furthermore, a fluid consisting of a gas such as air, a nitrogen gas, carbon dioxide, or a rare gas may be used. A fluid consisting of a gas or plasma that can etch the separation region may also be used.
The above separation method can be applied to the semiconductor substrate production method to enable the following methods:
1) A semiconductor substrate production method comprising the steps of preparing a first substrate comprising a porous single crystal semiconductor layer and a nonporous single crystal semiconductor layer sequentially stacked on a substrate; bonding the first substrate and a second substrate so as to provide a composite member having the nonporous single crystal semiconductor layer located inside; and jetting a fluid to the vicinity of the porous single crystal semiconductor layer in the composite member to separate the composite member at the porous single crystal semiconductor layer, or
2) a semiconductor substrate production method comprising the steps of implanting ions into a first substrate of a single crystal semiconductor at a predetermined depth to form an ion-implanted layer that can provide a microcavity layer; bonding the first substrate and a second substrate via an insulating layer so as to provide a composite member in which the ion-implanted surface of the first substrate is located inside; and jetting a fluid against the vicinity of the ion-implanted layer of the composite member to separate the composite member at the ion-implanted layer. This invention thus provides the semiconductor substrate production method that can solve the conventional problems.